Syria's main Kurdish forces declared today that they have successfully cleared areas east of the Euphrates river of Islamic State militants, with help from the US-led coalition and Russian forces.

The announcement comes after US Defence Secretary James Mattis said Washington will stop arming the YPG as offensive operations come to an end.

Noureddine Mahmoud, spokesman for the People's Defence Units, known as the YPG, and the backbone of the US-backed Syrian Democratic Forces, praised US and Russian air and logistical support as well as ground operations coordination.

He said his forces are ready to form joint operation rooms with the different partners to complete the fight against IS.

"We hope for an increase of the support and ensuring air protection and necessary cover," Mahmoud said at a press conference in al-Salihiya, a town in Deir el-Zour province today.

The press conference was attended by a Russian general from the Russian base in Hmeimeem, in western Syria.

The SDF have been battling IS fighters east of the Euphrates river in Deir el Zour province since September. In a separate campaign, Syrian government forces, backed by Russia and allied Iranian-backed militia, have been chasing IS militants on the other side of the river.

Russia and the US kept in contact, in so-called "de- confliction" talks, to prevent clashes between the two forces. US officials said communication was also maintained on the ground, including in meetings between SDF commanders and their counterparts on the government side.

Mahmoud also praised the role tribal leaders played in the fight against IS in the oil-rich Deir el-Zour province, where tribesmen play an integral role in consolidating power there.

The extent of SDF cooperation with Russian forces and their Syrian allies, however, is not clear.

The Kurdish-led forces now control nearly 25 per cent of Syrian territories and important oil resources after their battlefield successes in northern and eastern Syria.

But they are landlocked and economically dependent on areas controlled by the Syrian government, as well as on borders controlled either by Iraqi forces or the Syrian government.

Meanwhile, building relationships with the tribal leaders in Deir el-Zour is an essential part of any effort to continue to hold ground. Arab tribes are skeptical of the Kurdish-led forces, although they have previously worked together, with US support.

But Washington's support for the SDF has come into question after the fall of the IS de-facto capital of Raqqa and the near collapse of the militants in Syria.

Turkey, which considers the dominant Kurdish group in the force an extension of its own insurgent group, has protested Washington's reliance on the SDF, as well as the provision of heavy weaponry to its fighters, in the fight against IS.

On his way to Cairo, Mattis told reporters Friday that the US will stop arming the Kurdish forces, turning to stabilisation efforts and supporting the diplomatic process in Syria.

Mattis added that the SDF "are changing their stance as they come to the limits of where they are going."

Also today, the Russian Defence Ministry said six of its long-range Tu-22 bombers had carried out raids against IS targets in Deir el-Zour province.

A ministry statement said the bombers took off from Russia and were accompanied by fighter jets from Russia's air base in Hmeimeem.

The targets included ammunition warehouses and equipment clusters, the statement said.

Separately, Syrian activists and a war monitor said airstrikes in the rebel-held suburb of eastern Ghouta, near the capital Damascus, killed at least 18, including at least five children. The Ghouta Media Center and the Syrian Observatory for Human Rights said the airstrikes hit at least four neighbourhoods in the suburb, leaving scores injured.

Eastern Ghouta has been under a tightening siege since 2013 and is already facing a humanitarian crisis, including the highest recorded malnutrition rate since the conflict began in 2011. Some 400,000 civilians, half of them children, are believed trapped there.

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